I assume it depends on the officer who is responsible to do that and the number of applicants they have to assign the COS. I am still waiting for my COS. they said it can take up to 5 working day to inform the HR and then they have to fill an online form to get the reference number! we only have 5 ...

How did you manage to get your COS confirmation that time! It seems very late for HR to stay in their office
Congrats and good luck with Tier 2 visa!